Fig. 1

Overview of rice-Xoo interaction with a focus on effectors during pathogenesis. The Xoo-rice interaction is an example of a complex multi-layered arms race between the pathogen and host with effectors playing remarkable roles in determining the pathogenicity. Xoo gains access to the plant cellular contents through digesting the cell wall. This is achieved via secreting an array of cell wall degrading enzymes (CWDEs, shown as different coloured pie shapes) through the type II secretion system (1). The damage-associated molecular patterns (DAMPs) from degradation products of CWDEs and pathogen-associated molecular patterns (PAMPs) are sensed by specific receptors at the plasma membrane (2). This activates downstream signalling cascades (3) such as MAPK signalling leading to activation of transcription factors and upregulation of defense genes, resulting in defense responses such as callose deposition, programmed cell death, and release of ROS (4). The effector proteins secreted via the type III secretion system are directly delivered into the plant cell cytoplasm (5). These effectors consist of transcription activator-like (TAL) effectors, which are DNA binding proteins that upregulate plant genes leading to further susceptibility (6) (Classical example is SWEET gene upregulation in Xoo-rice interaction). Another class of T3S effectors—non-TAL effectors -are involved in dampening the immune responses by targeting defense signalling pathways, working directly or indirectly by binding to plant proteins (7). The plant counters these effectors using multiple mechanisms. This involves the executor R genes whose transcription is activated by TAL effectors leading to strong immune response and thus resistance (8), and by resistance proteins that target effectors directly or indirectly (9)